The invention relates generally to superconductors, and in particular to system and method for quench and over-current protection of a superconductive coil of an electric machine.
A superconductor is a material that will conduct electricity with no electrical resistance. Superconductivity occurs in certain materials when the material is cooled below a critical temperature. In conventional commercial power generation and transmission systems, such as generators, motors, and transformers, the amount of electrical current that flows through the systems may be significant. Because these conventional systems have electrical resistance, a significant amount of power that flows through the system is consumed as resistive heating. The power lost to resistive heating reduces the efficiency of the power generation system. Consequently, superconductors have been introduced for use in commercial power generation and transmission systems. For example, superconductive rotor coils have been developed for use in the rotors of electric machines. Electricity flowing through the superconductive rotor coil is used to produce a magnetic field. Because the superconductive rotor coil has no electrical resistance, the superconductive rotor coil is able to produce the magnetic field with no loss of power due to resistive heating.
However, there is a limit on the current a superconductor can carry and remain superconducting, known as the critical current. The critical current is a strong function of the temperature of the superconductor and the magnetic field. As the current flowing through the superconductor and the temperature changes during operation of the superconductor, the critical current also changes over time. If the current flowing through the superconductor exceeds the lowest critical current of the coil, a portion of the superconductor loses its superconductivity and enters a normal resistive state. The portion of the coil that is in the normal resistive state will cause resistive heating to occur in the superconductor. If the resistive heating of the superconductor is allowed to continue, the superconductor may enter a state of irreversible thermal runaway, known as a quench.
The quench condition may lead to damage of the superconductor. For example, in a superconductive rotor coil, a sufficient temperature gradient may be generated in the coil that will cause differential expansion to occur. The differential expansion may, in turn, lead to strain related damage in the coil. Therefore, it is desirable to determine the critical current of the superconductor during operation and to remove or reduce the current flowing through the superconductor during an over-current or a quench condition.
Quenching in the superconductive coil may be detected by measuring a voltage developed across the coil. However, this method of detecting quenching is problematic in electric machines that generate electrical noise because the noise causes large inductive voltages to be generated across the coil during normal operation, thus making it difficult to determine when quenching is actually occurring in the coil.
Similarly, temperature sensors may be used to monitor the coil temperatures. The magnetic energy is dumped from the coil when the detected coil temperature exceeds a predetermined limit. But it is difficult to locate the temperature sensors at the hot spot during a quench. Therefore this technique requires multiple temperature sensors to be placed at many locations to be effective.
Accordingly, a technique that enables an over-current condition or a quench condition to be detected in a superconductor is desirable. In addition, a technique that enables the superconductor to be protected from damage caused by an over-current or a quench condition is also desirable.